Tighten unit parsing in internal values

Interval values now generate an error when the user has multiple
consecutive units or a unit without a value.  Previously, it was
possible to specify multiple units consecutively which is contrary to
what the documentation allows, so it was possible to finish with
confusing interval values.

This is a follow-up of the work done in 165d581f14.

Author: Joseph Koshakow
Reviewed-by: Jacob Champion, Gurjeet Singh, Reid Thompson
Discussion: https://postgr.es/m/CAAvxfHd-yNO+XYnUxL=GaNZ1n+eE0V-oE0+-cC1jdjdU0KS3iw@mail.gmail.com

src/backend/utils/adt/datetime.c

@@ -3278,6 +3278,7 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 {
 	bool		force_negative = false;
 	bool		is_before = false;
+	bool		parsing_unit_val = false;
 	char	   *cp;
 	int			fmask = 0,
 				tmask,
@@ -3336,6 +3337,7 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 					itm_in->tm_usec > 0)
 					itm_in->tm_usec = -itm_in->tm_usec;
 				type = DTK_DAY;
+				parsing_unit_val = false;
 				break;
 
 			case DTK_TZ:
@@ -3373,6 +3375,7 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 					 * are reading right to left.
 					 */
 					type = DTK_DAY;
+					parsing_unit_val = false;
 					break;
 				}
 
@@ -3562,10 +3565,14 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 					default:
 						return DTERR_BAD_FORMAT;
 				}
+				parsing_unit_val = false;
 				break;
 
 			case DTK_STRING:
 			case DTK_SPECIAL:
+				/* reject consecutive unhandled units */
+				if (parsing_unit_val)
+					return DTERR_BAD_FORMAT;
 				type = DecodeUnits(i, field[i], &uval);
 				if (type == IGNORE_DTF)
 					continue;
@@ -3575,6 +3582,7 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 				{
 					case UNITS:
 						type = uval;
+						parsing_unit_val = true;
 						break;
 
 					case AGO:
@@ -3607,6 +3615,10 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 	if (fmask == 0)
 		return DTERR_BAD_FORMAT;
 
+	/* reject if unit appeared and was never handled */
+	if (parsing_unit_val)
+		return DTERR_BAD_FORMAT;
+
 	/* finally, AGO negates everything */
 	if (is_before)
 	{

src/test/regress/expected/interval.out

@@ -1796,3 +1796,12 @@ SELECT INTERVAL '2 minutes ago 5 days';
 ERROR:  invalid input syntax for type interval: "2 minutes ago 5 days"
 LINE 1: SELECT INTERVAL '2 minutes ago 5 days';
                         ^
+-- consecutive and dangling units are not allowed.
+SELECT INTERVAL 'hour 5 months';
+ERROR:  invalid input syntax for type interval: "hour 5 months"
+LINE 1: SELECT INTERVAL 'hour 5 months';
+                        ^
+SELECT INTERVAL '1 year months days 5 hours';
+ERROR:  invalid input syntax for type interval: "1 year months days 5 hours"
+LINE 1: SELECT INTERVAL '1 year months days 5 hours';
+                        ^

src/test/regress/sql/interval.sql

@@ -586,3 +586,7 @@ SELECT extract(epoch from interval '1000000000 days');
 -- "ago" can only appear once at the end of an interval.
 SELECT INTERVAL '42 days 2 seconds ago ago';
 SELECT INTERVAL '2 minutes ago 5 days';
+
+-- consecutive and dangling units are not allowed.
+SELECT INTERVAL 'hour 5 months';
+SELECT INTERVAL '1 year months days 5 hours';